In recent years, nonvolatile memory devices using a nonvolatile memory element such as a ferroelectric capacitor or variable resistance element have attracted attention as large-capacity and high-speed nonvolatile memory devices to be used in portable digital devices and the like.
Variable resistance elements include two types, namely, the unipolar (or monopolar) variable resistance element and the bipolar variable resistance element. The unipolar (or monopolar) variable resistance element has a characteristic of being able to change from the high resistance state to the low resistance state or from the low resistance state to the high resistance state according to two drive voltages of the same polarity. The bipolar variable resistance element has a characteristic of being able to change from the high resistance state to the low resistance state or from the low resistance state to the high resistance state according to two drive voltages of different polarities.
With the unipolar variable resistance element, a unidirectional diode which makes use of only the non-linear voltage-current characteristics of one voltage polarity can be used as a current control element, and thus the structure of a memory cell including a resistance element and a current control element can be simplified.
On the other hand, with the bipolar variable resistance element, a bidirectional diode which makes use of the non-linear voltage-current characteristics of both voltage polarities is typically used as a current control element. The bipolar variable resistance element has a high operating speed because the resetting operation for changing the variable resistance element to the high resistance state and the setting operation for changing the variable resistance element to the low resistance state can be performed using electric pulses of a short pulse width.
A nonvolatile memory device that is well known as a nonvolatile memory device using a variable resistance element has, arrayed in a matrix, memory cells of the so-called 1T1R type in which a MOS transistor and a variable resistance element are connected in series. For example, Patent Literature (PTL) 1 discloses a 1T1R nonvolatile memory device using a bipolar variable resistance element comprising copper oxide (Cu2O).
Furthermore, a nonvolatile memory device that is well known as a nonvolatile memory device using a variable resistance element has, arrayed in a matrix, cross-point memory cells of the so-called 1D1R type in which a diode and a variable resistance element are connected in series.